gzip


NAME
  gzip, gunzip, zcat - compress or expand files

SYNOPSIS
  gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ name ... ]
  gunzip [ -acfhlLnNrtvV ] [-S suffix] [ name ... ]
  zcat [ -fhLV ] [ name ... ]

DESCRIPTION
  Gzip reduces the size of the named files using Lempel-Ziv
  coding (LZ77).  Whenever possible, each file is replaced by
  one with the extension .gz, while keeping the same ownership
  modes, access and modification times.  (The default
  extension is -gz for VMS, z for MSDOS, OS/2 FAT, Windows NT
  FAT and Atari.) If no files are specified, or if a file name
  is "-", the standard input is compressed to the standard
  output.  Gzip will only attempt to compress regular files.
  In particular, it will ignore symbolic links.

  If the compressed file name is too long for its file system,
  gzip truncates it.  Gzip attempts to truncate only the parts
  of the file name longer than 3 characters.  (A part is
  delimited by dots.) If the name consists of small parts
  only, the longest parts are truncated. For example, if file
  names are limited to 14 characters, gzip.msdos.exe is
  compressed to gzi.msd.exe.gz.  Names are not truncated on
  systems which do not have a limit on file name length.

  By default, gzip keeps the original file name and timestamp
  in the compressed file. These are used when decompressing
  the file with the -N option. This is useful when the
  compressed file name was truncated or when the time stamp
  was not preserved after a file transfer.

  Compressed files can be restored to their original form
  using gzip -d or gunzip or zcat. If the original name saved
  in the compressed file is not suitable for its file system,
  a new name is constructed from the original one to make it
  legal.

  gunzip takes a list of files on its command line and
  replaces each file whose name ends with .gz, -gz, .z, -z, _z
  or .Z and which begins with the correct magic number with an
  uncompressed file without the original extension.  gunzip
  also recognizes the special extensions .tgz and .taz as
  shorthands for .tar.gz and .tar.Z respectively.  When
  compressing, gzip uses the .tgz extension if necessary
  instead of truncating a file with a .tar extension.

  gunzip can currently decompress files created by gzip, zip,
  compress, compress -H or pack. The detection of the input
  format is automatic.  When using the first two formats,
  gunzip checks a 32 bit CRC. For pack, gunzip checks the
  uncompressed length. The standard compress format was not
  designed to allow consistency checks. However gunzip is
  sometimes able to detect a bad .Z file. If you get an error
  when uncompressing a .Z file, do not assume that the .Z file
  is correct simply because the standard uncompress does not
  complain. This generally means that the standard uncompress
  does not check its input, and happily generates garbage
  output.  The SCO compress -H format (lzh compression method)
  does not include a CRC but also allows some consistency
  checks.

  Files created by zip can be uncompressed by gzip only if
  they have a single member compressed with the 'deflation'
  method. This feature is only intended to help conversion of
  tar.zip files to the tar.gz format. To extract zip files
  with several members, use unzip instead of gunzip.

  zcat is identical to gunzip -c. (On some systems, zcat may
  be installed as gzcat to preserve the original link to
  compress.) zcat uncompresses either a list of files on the
  command line or its standard input and writes the
  uncompressed data on standard output.  zcat will uncompress
  files that have the correct magic number whether they have a
  .gz suffix or not.

  Gzip uses the Lempel-Ziv algorithm used in zip and PKZIP.
  The amount of compression obtained depends on the size of
  the input and the distribution of common substrings.
  Typically, text such as source code or English is reduced by
  60-70%.  Compression is generally much better than that
  achieved by LZW (as used in compress), Huffman coding (as
  used in pack), or adaptive Huffman coding (compact).

  Compression is always performed, even if the compressed file
  is slightly larger than the original. The worst case
  expansion is a few bytes for the gzip file header, plus 5
  bytes every 32K block, or an expansion ratio of 0.015% for
  large files. Note that the actual number of used disk blocks
  almost never increases.  gzip preserves the mode, ownership
  and timestamps of files when compressing or decompressing.


OPTIONS
  -a --ascii
	   Ascii text mode: convert end-of-lines using local
	   conventions. This option is supported only on some
	   non-Unix systems. For MSDOS, CR LF is converted to LF
	   when compressing, and LF is converted to CR LF when
	   decompressing.

  -c --stdout --to-stdout
	   Write output on standard output; keep original files
	   unchanged.  If there are several input files, the
	   output consists of a sequence of independently
	   compressed members. To obtain better compression,
	   concatenate all input files before compressing them.

  -d --decompress --uncompress
	   Decompress.

  -f --force
	   Force compression or decompression even if the file has
	   multiple links or the corresponding file already
	   exists, or if the compressed data is read from or
	   written to a terminal. If the input data is not in a
	   format recognized by gzip, and if the option --stdout
	   is also given, copy the input data without change to
	   the standard ouput: let zcat behave as cat. If -f is
	   not given, and when not running in the background, gzip
	   prompts to verify whether an existing file should be
	   overwritten.

  -h --help
	   Display a help screen and quit.

  -l --list
	   For each compressed file, list the following fields:

		   compressed size: size of the compressed file
		   uncompressed size: size of the uncompressed file
		   ratio: compression ratio (0.0% if unknown)
		   uncompressed_name: name of the uncompressed file

	   The uncompressed size is given as -1 for files not in
	   gzip format, such as compressed .Z files. To get the
	   uncompressed size for such a file, you can use:

		   zcat file.Z | wc -c

	   In combination with the --verbose option, the following
	   fields are also displayed:

		   method: compression method
		   crc: the 32-bit CRC of the uncompressed data
		   date & time: time stamp for the uncompressed file

	   The compression methods currently supported are
	   deflate, compress, lzh (SCO compress -H) and pack.  The
	   crc is given as ffffffff for a file not in gzip format.

	   With --name, the uncompressed name,  date and time  are
	   those stored within the compress file if present.

	   With --verbose, the size totals and compression ratio
	   for all files is also displayed, unless some sizes are
	   unknown. With --quiet, the title and totals lines are
	   not displayed.

  -L --license
	   Display the gzip license and quit.

  -n --no-name
	   When compressing, do not save the original file name
	   and time stamp by default. (The original name is always
	   saved if the name had to be truncated.) When
	   decompressing, do not restore the original file name if
	   present (remove only the gzip suffix from the
	   compressed file name) and do not restore the original
	   time stamp if present (copy it from the compressed
	   file). This option is the default when decompressing.

  -N --name
	   When compressing, always save the original file name
	   and time stamp; this is the default. When
	   decompressing, restore the original file name and time
	   stamp if present. This option is useful on systems
	   which have a limit on file name length or when the time
	   stamp has been lost after a file transfer.

  -q --quiet
	   Suppress all warnings.

  -r --recursive
	   Travel the directory structure recursively. If any of
	   the file names specified on the command line are
	   directories, gzip will descend into the directory and
	   compress all the files it finds there (or decompress
	   them in the case of gunzip ).

  -S .suf --suffix .suf
	   Use suffix .suf instead of .gz. Any suffix can be
	   given, but suffixes other than .z and .gz should be
	   avoided to avoid confusion when files are transferred
	   to other systems.  A null suffix forces gunzip to  try
	   decompression on all given files regardless of suffix,
	   as in:

		   gunzip -S "" *       (*.* for MSDOS)

	   Previous versions of gzip used the .z suffix. This was
	   changed to avoid a conflict with pack(1).

  -t --test
	   Test. Check the compressed file integrity.

  -v --verbose
	   Verbose. Display the name and percentage reduction for
	   each file compressed or decompressed.

  -V --version
	   Version. Display the version number and compilation
	   options then quit.

  -# --fast --best
	   Regulate the speed of compression using the specified
	   digit #, where -1 or --fast indicates the fastest
	   compression method (less compression) and -9 or --best
	   indicates the slowest compression method (best
	   compression).  The default compression level is -6
	   (that is, biased towards high compression at expense of
	   speed).

ADVANCED USAGE
  Multiple compressed files can be concatenated. In this case,
  gunzip will extract all members at once. For example:

		gzip -c file1  > foo.gz
		gzip -c file2 >> foo.gz

  Then
		gunzip -c foo

  is equivalent to

		cat file1 file2

  In case of damage to one member of a .gz file, other members
  can still be recovered (if the damaged member is removed).
  However, you can get better compression by compressing all
  members at once:

		cat file1 file2 | gzip > foo.gz

  compresses better than

		gzip -c file1 file2 > foo.gz

  If you want to recompress concatenated files to get better
  compression, do:

		gzip -cd old.gz | gzip > new.gz

  If a compressed file consists of several members, the
  uncompressed size and CRC reported by the --list option
  applies to the last member only. If you need the
  uncompressed size for all members, you can use:

		gzip -cd file.gz | wc -c

  If you wish to create a single archive file with multiple
  members so that members can later be extracted
  independently, use an archiver such as tar or zip. GNU tar
  supports the -z option to invoke gzip transparently. gzip is
  designed as a complement to tar, not as a replacement.

ENVIRONMENT
  The environment variable GZIP can hold a set of default
  options for gzip. These options are interpreted first and
  can be overwritten by explicit command line parameters. For
  example:
		for sh:    GZIP="-8v --name"; export GZIP
		for csh:   setenv GZIP "-8v --name"
		for MSDOS: set GZIP=-8v --name

  On Vax/VMS, the name of the environment variable is
  GZIP_OPT, to avoid a conflict with the symbol set for
  invocation of the program.

SEE ALSO
  znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1),
  unzip(1), compress(1), pack(1), compact(1)

DIAGNOSTICS
  Exit status is normally 0; if an error occurs, exit status
  is 1. If a warning occurs, exit status is 2.

  Usage: gzip [-cdfhlLnNrtvV19] [-S suffix] [file ...]
		  Invalid options were specified on the command line.
  file: not in gzip format
		  The file specified to gunzip has not been
		  compressed.
  file: Corrupt input. Use zcat to recover some data.
		  The compressed file has been damaged. The data up to
		  the point of failure can be recovered using
				  zcat file > recover
  file: compressed with xx bits, can only handle yy bits
		  File was compressed (using LZW) by a program that
		  could deal with more bits than the decompress code
		  on this machine.  Recompress the file with gzip,
		  which compresses better and uses less memory.
  file: already has .gz suffix -- no change
		  The file is assumed to be already compressed.
		  Rename the file and try again.
  file already exists; do you wish to overwrite (y or n)?
		  Respond "y" if you want the output file to be
		  replaced; "n" if not.
  gunzip: corrupt input
		  A SIGSEGV violation was detected which usually means
		  that the input file has been corrupted.

  xx.x%
		  Percentage of the input saved by compression.
		  (Relevant only for -v and -l.)
  -- not a regular file or directory: ignored
		  When the input file is not a regular file or
		  directory, (e.g. a symbolic link, socket, FIFO,
		  device file), it is left unaltered.
  -- has xx other links: unchanged
		  The input file has links; it is left unchanged.  See
		  ln(1) for more information. Use the -f flag to force
		  compression of multiply-linked files.

CAVEATS
  When writing compressed data to a tape, it is generally
  necessary to pad the output with zeroes up to a block
  boundary. When the data is read and the whole block is
  passed to gunzip for decompression, gunzip detects that
  there is extra trailing garbage after the compressed data
  and emits a warning by default. You have to use the --quiet
  option to suppress the warning. This option can be set in
  the GZIP environment variable as in:
	for sh:  GZIP="-q"  tar -xfz --block-compress /dev/rst0
	for csh: (setenv GZIP -q; tar -xfz --block-compr /dev/rst0

  In the above example, gzip is invoked implicitly by the -z
  option of GNU tar. Make sure that the same block size (-b
  option of tar) is used for reading and writing compressed
  data on tapes.  (This example assumes you are using the GNU
  version of tar.)

BUGS
  The --list option reports incorrect sizes if they exceed 2
  gigabytes.  The --list option reports sizes as -1 and crc as
  ffffffff if the compressed file is on a non seekable media.

  In some rare cases, the --best option gives worse
  compression than the default compression level (-6). On some
  highly redundant files, compress compresses better than
  gzip.